Fix xDS missing endpoint race condition.

This fixes the following race condition:
- Send update endpoints
- Send update cluster
- Recv ACK endpoints
- Recv ACK cluster

Prior to this fix, it would have resulted in the endpoints NOT existing in
Envoy. This occurred because the cluster update implicitly clears the endpoints
in Envoy, but we would never re-send the endpoint data to compensate for the
loss, because we would incorrectly ACK the invalid old endpoint hash. Since the
endpoint's hash did not actually change, they would not be resent.

The fix for this is to effectively clear out the invalid pending ACKs for child
resources whenever the parent changes. This ensures that we do not store the
child's hash as accepted when the race occurs.

An escape-hatch environment variable `XDS_PROTOCOL_LEGACY_CHILD_RESEND` was
added so that users can revert back to the old legacy behavior in the event
that this produces unknown side-effects. Visit the following thread for some
extra context on why certainty around these race conditions is difficult:
https://github.com/envoyproxy/envoy/issues/13009



This bug report and fix was mostly implemented by @ksmiley with some minor
tweaks.

Co-authored-by: Keith Smiley <ksmiley@salesforce.com>